Daily Image
24-10-2007
The Look and Feel of LOFAR
| Submitter: | Sarod Yatawatta and Jan Noordam |
| Description: | This is a simulated noise-free image, as seen from LOFAR CS1 at an observing frequency of about 60 MHz. The utilised sky model is limited to ~50 of the brightest sources, leaving out the brightest two, Cas A and Cyg A. During the 24 hours of the 'observation', some of the sources were below the horizon for some of the time. An analytic model of the station response beam was used. Because there is no noise, the sidelobe pattern of the Point Spread Functions (PSF) around the various sources is clearly seen in the top-left image. The PSF's are rather different from the neat concentric rings of the WSRT PSF. A large part of LOFAR processing consists of getting rid of ('deconvolving') these patterns. The (prototype) LOFAR CS1 array consists of 16 elements, each of which has two perpendicular dipoles, X and Y. All 16 dipole pairs are oriented in the same way. During observation, the signals received by each dipole are combined (correlated) with those of all other dipoles, so that we end up with 4 different data-sets: XX, XY, YX and YY. These are combined to form the 4 images above: XX + YY -> I (total intensity) XX - YY -> Q (linear polarization) XY + YX -> U (linear polarization) XY - YX -> V (circular polarization) However, these transformation rules are only valid when the dipoles are perpendicular to the incoming radiation. This is always the case for the WSRT, where the parabolic dishes 'follow' the rotating sky. But for the fixed, horizontal LOFAR dipoles, it is true only for the zenith direction, which contains different sources all the time. Therefore, the images above cannot be interpreted in the usual way. But they contain a lot of very interesting features for those who are learning to develop a feeling for the new kind of 'Look and Feel' of a telescope like LOFAR, which is so different from the one(s) we are used to. |
| Copyright: | ASTRON |
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